Nanoscale gold boosts infrared detection

TROY, N.Y. – For the past decade, state-of-the-art infrared photodetectors have been based on MCT, or mercury cadmium telluride, technology, which offers strong signal sensing but is associated with various drawbacks, such as its suboptimal signal-to-noise ratio and the long exposure times associated with low-signal imaging.

Quantum dot-based infrared detectors, the alternative, are less sensitive to light, especially in the far field. However, researchers at Rensselaer Polytechnic Institute, led by physics professor Shawn-Yu Lin, have developed a device dubbed a “microlens” that uses the unique properties of nanoscale gold to more than double the detectivity of this sensor type without increasing the noise.

This “microlens” is coated with nanoscale gold and intensifies infrared detection by optimizing the interaction between surface plasmon resonance and quantum dots. It was developed at Rensselaer Polytechnic Institute by a group led by professor Shawn-Yu Lin. Courtesy of RPI.
The microlens is a flat structure covered with a layer of gold about 50 nm thick. The gold’s surface plasmons – free-floating electrons – “squeeze” incoming light into the device’s numerous, regularly spaced holes, each measuring about 1.6 µm in diameter and 1 µm deep. Within the hole, the indium arsenide quantum dots “trap” the photons, concentrating them and strengthening the interaction between the light and the semiconductor material.

“We have shown that you can use nanoscopic gold to focus the light entering an infrared detector, which in turn enhances the absorption of photons and also enhances the capacity of the embedded quantum dots to convert those photons into electrons. This kind of behavior has never been seen before,” Lin said.

The result is an electric field up to 400 percent stronger than the raw energy that enters the photodetector. The group’s new design resulted in a signal enhancement of 130 percent. The effect is comparable to covering each tiny hole on the device with a lens, but without the extra weight, and minus the hassle and cost of installing and calibrating millions of microscopic lenses, Lin said. He plans to continue honing the design both by widening the diameter of the surface holes and by placing the quantum dots more effectively.

“I think that, within a few years, we will be able to create a gold-based [quantum dot infrared photodetector] device with a twentyfold enhancement in signal from what we have today,” Lin said. “It’s a very reasonable goal and could open up a whole new range of applications from better night-vision goggles for soldiers to more accurate medical imaging devices.”

This technology could pave the way to quantum dot detectors that surpass MCTs in infrared detectivity. Low in cost and with the potential to be mass-produced, they could benefit applications ranging from homeland security and surveillance to medical diagnostics.

The study, titled “A Surface Plasmon Enhanced Infrared Photodetector Based on InAs Quantum Dots,” was published in the May 12, 2010, issue of Nano Letters.